Child BMI Calculator (5-19 Years): BMI Percentile by Age and Sex

What a BMI is

BMI is one number: weight in kilograms divided by height in metres, squared. It asks whether a child's weight is in proportion to their height, and nothing else.

For an adult it is read against fixed lines, 25 and 30. For a child those lines mean nothing. A healthy BMI is naturally low at five, dips through early childhood, then climbs through the school years, and it runs differently for boys and girls. So a child's BMI is placed on the WHO 5 to 19 growth reference for their exact age and sex and reported as a percentile: a child on the 60th percentile has a higher BMI than 60 in 100 children of the same age and sex.

Choose sex, then enter age in years (5 to 19), weight in kilograms and height in centimetres.

The healthy band, and where your child sits

Healthy is a wide band on purpose: roughly the 3rd to the 85th percentile. A child steady on the 20th is not underweight, and one on the 75th is not overweight. Above the 85th is overweight, above the 97th is obese, and below the 3rd is thinness.

The same weight and height give a different verdict at seven and at twelve, which is why the cut-offs in the chart move with age. Read your child's BMI against their own row and sex.

One reading matters less than the trend. A child tracking along their own percentile is usually fine whatever the number; a child crossing several lines up or down over months is the pattern worth a doctor's look.

What the number does not tell you

BMI is a screen, not a diagnosis. It uses only weight and height, so it cannot tell muscle from fat, and a stocky, muscular teenager can read as overweight while carrying little of it. Growth spurts throw it around too: the weight often arrives before the height jump.

The growth chart your paediatrician keeps, plotted over time, is the real tool. This is a between-visits check, useful for spotting a trend worth mentioning, not for labelling a child.

If the reading is outside the healthy band

Do not put a child on a weight-loss diet off the back of a calculator. Children are still growing, and restricting food can do real harm; for a child who is overweight the usual aim is to hold weight steady while they grow into it. That is a plan to make with your doctor.

Whole-family habits do more than any diet aimed at one child: home-cooked meals over packaged and fried food, water instead of soft drinks and packaged juice, real playtime, less screen time, and enough sleep. A reading in the thinness range deserves the same attention, especially if a child has been dropping across percentile lines, is often unwell, or is off their food.

When to see your doctor

See a doctor if the BMI sits in the overweight, obese or thinness band, if it has crossed percentile lines over recent months, or if the weight change comes with other signs: unusual tiredness or thirst, frequent illness, or a very early or very late puberty. Under five this reference does not apply at all — use the growth calculator's weight-for-height instead. And treat the category as information, not a label to repeat in front of the child.

Common questions